Probe heads for coordinate-measuring instruments have a movable part which carries the probe pin and its work-contacting probe-tip ball; upon ball contact with a workpiece, the probe pin is deflected out of its position of rest against the force of one or more springs, generally coil springs, during a work-contacting procedure. This deflection motion is necessary to protect the probe head from damage as a result of unavoidably overshooting the contact position, due to drive action in the coordinate measuring instrument. Probe heads for coordinate-measuring instruments are illustratively described in West German Pat. No. 2,347,633 and OS No. 2,743,665, corresponding to U.S. Pat. No. 4,136,458.
The design of probe heads requires, on the one hand, the largest possible free stroke for probe deflection, so that high travel speeds and short measurement times can be obtained. On the other hand, the probe head should be structurally as small as possible, to enable contact with even the workpiece locations which are most difficult to access.
It is difficult to simultaneously satisfy both these requirements, particularly if cables (e.g. electrical wiring) must be brought to the movable part of the probe head. This is necessary, for example, in the case of probe heads which, like the pressure-sensitive sensors described in West German Pat. No. 2,712,181 (U.S. Pat. No. 4,177,568), or like the probe head described in International Application WO81/01876, employ electrical components such as a piezo-oscillator; if such electrical components are arranged on the movable part itself, frequent large deflection movements in a very small space entail the danger of cable breakage. Furthermore, additional restoring forces are attributable to the cable itself; and as a result of these added forces, the precision of measurement obtainable with the probe head is reduced, or the function of the probe head can even be entirely destroyed.
Admittedly, it is known to lay cables in loops in order to assure low cable stresses, in the circumstance of relative movement between parts connected to the cable. However, such an arrangement requires large structural space.